The present invention is directed to a tool bit to be inserted into a tool bit chuck for hand tools used for chiseling, drilling and/or rotary percussion drilling. The tool bit has a circular chucking shank with at least one axially extending locking groove closed at its ends spaced apart in the axial direction and at least one rotary entrainment groove open at a free end of the shank.
Tool bits and tool bit chucks are disclosed in DE-PS 25 51 125 in which the chucking shank of the tool bit has one or two axially extending locking grooves closed in the axial direction as well as one or two rotary entrainment grooves open towards the free end of the chucking shank. The tool bit chuck is arranged to receive the tool bit and has radially displaceable locking members corresponding to the number of locking grooves, and the locking members are in the shape of balls or spheres. The locking members in cooperation with the locking grooves prevent the tool bit from falling out of the tool bit chuck. The locking members can be radially displaced, shifting out of the locking grooves, so that the tool bit can be removed from the chuck.
No particularly high loads are applied to these locking grooves and cooperating locking members, since, in operation, the tool bit positioned in the tool bit chuck is for all intents and purposes supported floatingly relative to the locking members, whereby the locking members do not transmit any forces worth mentioning when they cooperate with the locking grooves. It is only when the tool bit is pulled out of a borehole in a structural component that the locking members in cooperation with the locking grooves must assure the connection between the tool bit and the tool bit receptacle.
Very high loads are developed in the axially extending rotary entrainment grooves open at the free end of the chucking shank which engage corresponding rotary entrainment members or strips in the tool bit chuck. The rotary entrainment grooves along with the rotary entrainment members or strips carry the entire torque transmitted during operation of the tool.
The weakness of these known tool bits and tool bit chucks is the amount of wear of the rotary entrainment grooves and the rotary entrainment members or strips, especially at the flanks on the entrainment side, whereby an extraordinarily high wear occurs at the flank located upstream in the direction of rotation but facing away from the direction of rotation. The cause of such wear is the high torque transmitted and the continuous relative offset of the flanks of the rotary entrainment grooves against the flanks of the rotary entrainment members. This offset relationship occurs in particular from the effect of percussion or shock loads acting on the tool bit during chiseling or percussion drilling operations. Since such wear results in the rotary entrainment grooves in the tool bit being beaten or crushed to such an extent that a reliable torque transmission is no longer possible before the normal wear under proper operation in the working range of the tool bit takes place. Such wear results in the expensive replacement of the tool bit.